The present invention is generally directed to a method and apparatus for laminating a polymer film over the surface of an irregular substrate, particularly a substrate on which a plurality of integrated circuit chips are affixed. More particularly, the present invention relates to a packaging method for electronic integrated circuit chips, particularly very large scale integrated (VLSI) devices. Moreover, the method and apparatus of the present invention provides a polymer film overlay for not only retaining "microchips" on a substrate, but also provides a means for interconnecting these devices. Even more importantly, the method and apparatus of the present invention provide removable interconnection capabilities.
In the packaging of very large scale integrated circuit devices, a great deal of space is taken up by mechanisms for interconnecting one chip to an adjacent device. This makes the packaging of integrated circuit devices and electronic components based thereon larger than necessary. As a result of this, many individuals are involved in the development of so-called wafer-scale integration processes. However, the efforts expended in these directions have generally tended to be limited by the problem of yield. Because a certain number of chips or dies on a wafer are often found to be defective, the number of wafers that are produced that are completely usable is generally lower than is desired. Furthermore, there still exists the problem of interconnecting the various chips on a wafer and the concomitant problem of testing a large system, such as results when a number of highly complicated individual integrated circuit components are interconnected. Accordingly, it is seen that it would be very desirable to be able to construct wafer scale integrated circuit packages from individual, easily testable integrated circuit chips. It is to this end that the present invention is directed.
More particularly, the present invention is directed to a method and apparatus for applying a polymer film overlay. This film covers a plurality of integrated circuit chips disposed adjacent to one another on an underlying substrate. Furthermore, the polymer film of the present invention provides an insulative layer for interconnection of these individual circuit chips. A significant advantage of the system of the present invention is the ability to remove one or more of these interconnection layers so as to provide a multitude of arrangement and testing capabilities.
One of the generic problems sought to be solved by the system of the present invention is the laminating of a polymer film to an irregular substrate. Certain laminating methods suffer from several deficiencies that become extremely important when one is trying to dispose film material on integrated circuit devices. This is particularly true in those circumstances in which passivation layers have not yet been applied to such devices. One method of continuous lamination is so-called roll lamination in which a substrate to be laminated is passed between two heated rollers and the pinching effect of the rollers, combined with heat, laminates the film to the substrate. Several problems exist in this approach. The approach is adequate if the substrate is perfectly flat. However, the rollers cannot comply to an irregular surface, with the result being a poor lamination in low spots and overpressure effects in high spots. Also, a substrate with an irregular surface on the bottom side would damage the lower roller or the substrate. For example, pins coming down from the bottom of a package would be damaged. Another problem with roll lamination occurs when high temperatures are required to process high temperature adhesives. Compliant materials are not available with high temperature capabilities. Silicone materials seem to be the best known compliant substances for this purpose. However, this material tends to revert to a sticky goo at temperatures around 200.degree. C. A third problem also exists with this methodology since it is necessary that the top roller touch the material to be laminated. It is impossible then to prevent contamination on the top roller from contaminating the surface of the polymer being laminated. This problem is especially serious with rollers comprising silicone material. Silicone materials produce outgassing products which contaminate the top surface and prevent good adhesion when subsequent layers are applied to the top surface of the film being laminated.
A second method used in laminating involves the use of a vacuum bag. In this system, a bag formed of high temperature material is placed around the materials to be laminated. The bag is evacuated and the bag placed in an oven. There are several problems associated with the vacuum bag methodology, however. One problem is that it is not a continuous process. That is, material to be laminated must be placed in the bag, the bag evacuated, the bag placed in an oven for a period of time, and after lamination, the laminated piece is removed from the bag. Automatic feed of material is therefore precluded. The second problem with vacuum bag methodologies is that the pressure is limited to the outside air pressure, even in the face of a perfect vacuum. Still another problem with this method is that the vacuum bag touches the top of the material being laminated. Contamination from the bag can then be transferred to the material being laminated.
A third laminating method is the standard press lamination. In this method, the material to be laminated is placed between two heated platens which are forced together by means such as a hydraulic jack. This technique works if the materials to be laminated are flat. If the substrate to be laminated is irregular in shape, or if there are small particles on the surface of the substrate to be laminated, very high pressures where the particles are present will punch through the film to be laminated and no pressure is available at the low points on the substrate. The thinner the film, the greater this problem becomes. A partial solution to this problem is the use of compliant pads to absorb some of the pressure differential. However, compliant pads contaminate the top surface of the film to be laminated and, in high temperature situations, there are no suitable compliant pad materials available. An additional problem of press laminating is that it takes time and energy to cool and heat the relatively high thermal mass platens. The platens must be physically thick structures because they are required to take high pressures over a large area without bending. An additional problem with press laminating is the removal of air and by products of the heating of adhesives during the lamination process, since the pressure tends to trap air and byproducts and tends to form blisters.
Additional problems occur when the film to be laminated to a substrate is a thermoplastic. Unless the top press pad has a special release property, the thermoplastic often will stick as well to the top press pad as it does to the substrate. Another problem exists in that no compliant pad can provide the dual function of maintaining constant pressure on the thermoplastic film at the same time that it perfectly matches the contour of the underlying substrate. Accordingly, it is seen that certain laminating methodologies are inappropriate for use in facilitating the construction of wafer scale integrated circuit devices, as contemplated herein.